Low-res texture blurry
If you are using Blender Internal Render Engine:
In the Properties Window > Texture Tab > Image Sampling Panel > Filter
change it to Box.

Have in mind that this will not have any effect in the viewport preview, but it won't appear blurry in your final render.

If you are in Cycles/EEVEE Render Engine:
In the Node Editor, under your material node tree navigate to your image texture node and set the second pull-down menu Interpolation to Closest